Variable frequency oscillators are extensively used to provide a signal with a frequency which can be varied over a predetermined range. An exemplary use for such an oscillator is in a phase phase-locked-loop where the frequency of the signal from the oscillator is forced to follow the frequency, or a multiple of the frequency, of an input signal to the phase-locked-loop by varying a voltage applied to a control input of the variable frequency oscillator. Typical variable frequency oscillators, such as voltage controlled oscillators (VCOs) or voltage controlled crystal oscillators (VCXOs), may have three separate components: a frequency determining network, a voltage controlled variable capacitor (VVC) and an oscillator circuit. The frequency determining network is either a high quality (high "Q") tank circuit, or crystal resonator, which, in combination with the VVC, determines the output frequency of the VCO or VCXO. The VVC is a two terminal device which changes its capacitance in response to an externally supplied control voltage impressed across its terminals. The change in capacitance by the VVC "pulls" the resonant frequency of the tank circuit or crystal resonator and, hence, varies the output frequency of the oscillator. The oscillator circuit is typically thought of as a two terminal (one port) circuit, utilizing bipolar or metal-oxide-semiconductor (MOS) technology, providing the necessary gain and feedback to achieve and sustain oscillation. But having a VVC separate from the oscillator circuitry increases the cost and reduces both the manufacturing yield and reliability of a variable oscillator utilizing a separate VVC.
In VCXOs, the frequency determining network, a crystal resonator, is wired in series with the VVC and the oscillator circuitry. However, the VVC is not integrated onto the same substance or epitaxial layer on a substrate (hereinafter referred to as a semiconductor body) as the oscillator circuitry since the structure of VVC of the prior art has only one terminal thereof available for coupling to the crystal or oscillator circuitry; the remaining terminal is coupled to the semiconductor body (ground). One such VVC is illustrated in "Device Electronics for Integrated Circuits", by R. S. Muller and T. I. Kamins, 1977, p. 344, FIG. P7.7(a). As shown, the VVC has one terminal thereof being the conductive region insulated from the semiconductor body by an oxide layer; the body itself being the remaining terminal. Extensive evaluation of the ideal characteristics of this type of VVC is described in "Ideal MOS Curves for Silicon", by A. Goetzberger, Bell System Technical Journal, September, 1966, pp. 1097-1122. Further, a description of the operation of a similar VVC is described in detail in "Device Electronics for Integrated Circuits" on pp. 314-317. But for purposes here the operation thereof is described briefly herein. As the voltage applied to the terminal exceeds a predetermined threshold voltage, the body directly beneath the electrode becomes depleted of carriers (depletion) and becomes non-conductive. The depth of the depletion layer varies with the voltage on the electrode; the capacitance varying inversely with the depth of the depletion region and, therefore, inversely with the applied voltage. This is analogous to the "movable" plate (the interface between the depletion layer and the undepleted portion of the body) of a mechanical air-dielectric variable capacitor varying in distance from the "fixed" plate thereof (the conductive layer). This type of VVC has the drawbacks of high series resistance due to the body having relatively high resistivity (ranging from several hundred to several throusand ohm/square) and having one terminal of the VVC coupled to ground (the semiconductor body.) However, in VCXOs utilizing a VVC, it is preferable to have both terminals of the VVC isolated from ground for maximum circuit flexiblity in determining VCXO center frequency. Further, a low series resistance for the VVC gives the best frequency stability and highest frequency performance. To achieve this, the VVC is physically separated from the oscillator circuitry and is usually a hyper-abrupt p-n junction diode. Even though it is possible for such a diode to be integrated with the oscillator circuitry, the processing steps necessary for the fabrication of the diode are not readily compatible with the processing steps utilized to fabricate the oscillator circuitry; extra processing steps are required which increases the cost of the fabrication thereof to such an extent that VCXOs constructed with the hyper-abrupt diode in the same semiconductor body as the oscillator circuitry costs more than separate VVC and oscillator circuitry designs. Another type of VVC is a conventional MOS transistor with one terminal being the gate electrode thereof and the other terminal being the drain or source (or both) electrodes thereof. Operation of such a VVC is similar as that described above. However, the capacitance variation possible with this structure is usually insufficient for variable oscillators except those operating over a very narrow frequency range, making them unsuitable for general purpose VCOs or VCXOs.